The embodiments of this invention generally relate to online flow control along the span of a high aspect ratio slot jet with applications to airknives as used in industry to apply coatings, dry coatings or to control the thickness of coatings. Most problematic is controlling the coating thickness distribution in the hot-dip galvanizing industry, where excess zinc coating of sheet steel is an expensive waste of material. With frequent changes in sheet thickness-width-and-speed, together with changes in furnace temperature, zinc pot temperature-and-chemical composition, coating thickness control is an ongoing problem for the operator. Currently the operator's options are limited to changing sheet speed, airknife supply pressure, distance between slot jet and sheet, and the blowing angle onto the sheet. To prevent coating edge buildup with associated coiling problems of the finished sheet goods and to improve coating uniformity, the operator has the option to change offline, the “bow” setting in the slot jet nozzle lips. To change the bow setting offline requires taking the slot jet to a machine shop for nozzle lip gap adjustment. A “bow” setting in the slot jet nozzle lip gap, is used to increase the jet mass flow rate or momentum, and thereby the wiping action, towards the edges of the sheet where the lack of flow blockage deflects the flow outward, thereby locally reducing the stagnation pressure on the sheet and thus wiping action.
Airknife technologies from the 1990's incorporated on-line controllable internal swiveling elbows to produce a fan shaped outflow angle of the airknife slot jet. This method proved to be effective in reducing edge build-up. However this mechanism was complex with numerous moving parts and often unreliable. After fixing the position of the outflow generating elbows, such airknives remained in service over the past two decades. Other operators resort to: (1) fences placed near the edges of the sheet to minimize edge build-up and coiling problems or (2) a bow-like setting in the airknife lips, to increase the mass flow rate and thus wiping action near the sheet edges. Figures from U.S. Pat. No. 5,683,514 are shown in FIG. 5 to illustrate the obtainable fan-like outflow pattern by adding swiveling elbows inside the airknife inlet plenum. The ever increasing cost of coating materials increases the demand for new technologies with online control over coating thickness distribution. This is likely to be in the form of online control over the distribution of any or all local mass flow rate or velocity or outflow angle along the length (span) of the slot jet of an airknife.